Thanks for the tip...until now i didn't visit the show place so am not clear about the lights and all
by using the 70-200mm for the far shut will the extarnal flash be enough
btw what about using the 18-250mm???

24-70mm 2.8 (if you're gonna be somewhat close to the runway.
Or...70-200mm 2.8 if you're obviously not going to be too close to the runway.

Fashion shows usually have low light in the room itself and the stage is blasting with light to illuminate the models so everyone can see them clearly.

You're best option is to go with a fast lens so that you will be able to open the aperture wide enough to get a blurry background, use a low iso and a fast shutter speedf to get all the action without blurring the models movements when they catwalk down the strip.